PALB2 is tumor suppressor protein that physically and functionally links BRCA1 and BRCA2, the two major breast cancer suppressors. The 3 proteins form a BRCA complex and function together in DNA double strand break (DSB) repair and cell cycle checkpoint control following DNA damage. These functions are critical for the maintenance of genome stability and suppressing tumorigenesis. We and others have shown that BRCA1 functions upstream of PALB2 and BRCA2. However, how BRCA1 regulates PALB2 in various DSB repair pathways remain poorly understood, and the mechanism how the 3 proteins promote checkpoint response remains unknown. Moreover, how much of BRCA1's tumor suppressive function is transmitted by PALB2 and to what extent PALB2's in vivo function depends on BRCA1 are also unknown. In this project, we will delve into the molecular underpinnings of the BRCA1- PALB2-BRCA2 DNA damage response network to understand key regulatory mechanisms that govern DSB repair efficiency, pathway choice and checkpoint control. Moreover, we will also use our newly generated Palb2 knockin mouse, in which the endogenous PALB2 is unable to bind BRCA1, to explore the etiology and tissue specificity of PALB2- and BRCA1-associated cancers. In Aim1, we will investigate the role and mechanism of the BRCA1-PALB2 interaction in homologous recombination (HR) and single strand annealing (SSA). In Aim2, we will define the mechanism of BRCA1, PALB2 and BRCA2 in the G2/M checkpoint control. In Aim3, we will use the above mouse model to explore the role of the BRCA1-PALB2 complex formation in the DNA damage response in vivo and in tumor suppression in different tissues.